The present invention relates to methods for disinfecting airstreams containing microorganisms by electrostatic precipitation. More particularly, the invention relates to methods in which at least one of the grounded collection plates of an electrostatic precipitator in contact with an airstream containing microorganisms is coated with a photocatalyst having a predetermined band gap energy and illuminated with photons having a wavelength corresponding to the band gap energy of the photocatalyst. The present invention also relates to electrostatic precipitators having collection plates coated with a photocatalyst having a predetermined band gap energy and a light source positioned to illuminate the photocatalyst coating with photons having a wavelength corresponding to the band gap energy of the photocatalyst.
Americans spend 90% of their time indoors, and while indoors, are exposed to a variety of airborne contaminants such as volatile organic compounds (VOC's), radon and biological organisms. In a 1980 study, the Environmental Protection Agency (EPA) concluded that indoor air pollution posed a greater health risk than outdoor air pollution. Indoor air contamination is estimated to cause significant increases in medical costs and a decline in worker productivity.
Pollutant levels from individual indoor sources may not pose a significant risk by themselves, however, many buildings have more than one source that contributes to indoor air pollution. Illnesses resulting from such indoor pollutants are sometimes known as the "sick building syndrome." Common causes of indoor air pollution are unwanted particulate matter, unwanted chemical substances, volatile organic compounds (VOC's) and microbial contaminants. In the first two cases, conventional technology can often ameliorate the contamination by filtration and adequate ventilation. The problem of volatile organic compounds and microbiological contamination creates a more serious obstacle, not easily solved by filtration or ventilation.
Because so many Americans spend a great deal of time in offices and buildings with various mechanical, heating, cooling, and ventilating systems, such systems pose a significant health risk of biological contamination. In recent years, biological problems in indoor environments have received considerable attention. The 1976 Legionnaires' disease outbreak in Philadelphia is probably the most publicized case of illness caused by indoor pollutants.
Biological contamination of building systems can include contamination by bacteria, molds, and viruses. There are a variety of places in a building's heating, mechanical, air-conditioning, refrigeration, and other air and water circulating systems that can become a breeding ground for biological contaminants. Moreover, the forced air of a building's heating, ventilation and air-conditioning systems can distribute the contaminants throughout the building, thus compounding the contamination.
UV disinfection has been widely used in the past to destroy biological contaminants and toxic chemicals. Such UV treatment has worked well for disinfection, but the indoor environment may also be contaminated with low level toxic chemicals such as formaldehyde, styrene, and toluene. Ultraviolet energy alone has proven ineffective in degrading these chemicals. For instance, U.S. Pat. No. 5,045,288 to Raupp and Dibble, and U.S. Pat. Nos. 4,892,712, 4,966,759 and 5,032,241 to Robertson and Henderson use UV to treat fluids and gases that contain pollutants.
An alternative that has gained much attention is photocatalytic oxidation, which involves the use of a photocatalyst such as TiO.sub.2 for the total destruction of both hydrocarbons and microorganisms. Patela, Antibacterial Effect Of Catalyzed Radiation, Masters Thesis, University of Florida, Gainesville (1993), reports powdered TiO.sub.2 to be capable of killing Serratia marcescens after irradiation for 60-120 minutes in water. Saito et al., J. Photochem. Photobiol. B:Biol., 14, 369-79 (1992); Matsunaga, J. Antibact. Antifungic. Agents, 13, 211-20 (1985); Nagane et al., J. Dent. Res., 68, 1696-7 (1989) and Moriaka et al, Caries. Res., 22, 230-1 (1988), report TiO.sub.2 to be capable of killing E. coli and Lactobacillus acidophilus after aeration for 60-120 minutes in water. Wang et al., Proceedings of the First International Conference on TiO.sub.2 Photocatalytic Purification and Treatment of Water and Air (London, Ontario, Canada, Nov. 8-13, 1992) pp. 733-39; Wang et al., Proc. AWWA Conf. (San Antonio, Tex., 1993); Savat et al., J. Catalysis, 127, 167-77 (1991) and Anderson et al., Further Catalytic Purification of Water and Air, 1, 405-20 (1993), report the gas phase detoxification of trichloroethylene (TCE) and other organic contaminants.
Co-pending and commonly owned U.S. patent application Ser. No. 08/647,070, filed May 9, 1996, discloses that the effectiveness of photocatalytic oxidation increases significantly when the relative humidity of the airstream is maintained at levels greater than about 40%. Under such conditions, it is possible to destroy the microorganisms in an airstream; however, the process requires a certain minimum residence time for complete effectiveness.
Electrostatic precipitators can trap microorganisms but cannot destroy them. Therefore, the hazard of such microorganisms remains. Specifically, electrostatic precipitators retain microorganisms on the grounded collection plate, which must be cleaned periodically to maintain the effectiveness of the precipitator at removing microorganisms from airstreains.
There remains a need for more efficient airstream disinfection devices.